Java polymorphism-16 days study notes

Polymorphism

• Polymorphic compilation: Type: Scalable
• That is, the same method can adopt a variety of different behaviors depending on the sending object
• The actual type of an object is determined, but there are many types of references that can point to the object

Conditions for the existence of polymorphism

• Inheritance

• Subclass overrides the method of the parent class

• The parent class reference points to the child class object

Note: Polymorphism is the polymorphism of the method, there is no polymorphism in the attribute

1. Polymorphism is the polymorphism of the method, the attribute is no polymorphism

2. The parent class and the child class are inherited and connected! ClassCastExcepetion;

3. Existence conditions: inheritance relationship, methods need to be rewritten, and parent class references point to subclass objects! Father f1 = new Son();

   But some methods cannot be overridden

   • static is a static method, which belongs to a class, not an instance
   • final constant
   • private can't

4. (S1 has run, s2 has run, and both have run. The program only knows who to call when it is running)

​ For us, we can execute whichever type is on the left, and only execute subclasses when there are two types .

Application

package com.oop;

import com.oop.Demo05.Person1;
import com.oop.Demo05.Stunden1;

//一个项目应该只有一个main方法
public class Application {

    public static void main(String[] args) {
        //类型之间的转换   父亲     儿子

      //高                  低     低转高 向上转型自动升型，
       Person1 p1 = new Stunden1();

       //s1将这个对象转换为Student类型，我们就可以使用Student1类型的方法
          //    低                   s1为高  高到低 向下转型 强制
           Stunden1 s1  =  (Stunden1)p1;//强制转换
           s1.go();
           p1.move(); //还是输出子类的重写方法，丢失父类的原始方法
        //((Stunden1)s1).go(); 也可以这样写之间强制加调用
        //同级的子类之间不能强制转换
       // Teacher t1 = (Stunden1)s1; 编译不通过s1已经被转换为tudent1S，


        Stunden1 s2 = new Stunden1();//可以调用本身的方法
        s2.go();;



    }
    /*
    1.父类引用指向子类的对象
    2.把子类转换为父类，向上转型，自动转型，丢失子类中原本可以之间调用的特有方法
    3.父类转换为子类要强制转换，向下转型，丢失父类被子类所重写的方法。
    4.方便方法的调用，减少重复的代码

    抽象 ： 封装，继承，多态


           */
}

Person1

package com.oop.Demo05;

public class Person1 {
    
    

public void move(){
    
    
    System.out.println("移动");
}
}
/*
//Object > String
        //Object > Person1 > Teacher
        //Objiec > Person1 >Student1

        Object object = new Stunden1();

        //System.out.println(X instanceof Y);//判断是否编译通过

        System.out.println(object instanceof Stunden1);//ture
        System.out.println(object instanceof Person1);//ture
        System.out.println(object instanceof Object);//ture
        System.out.println(object instanceof Teacher);//false
        // Object > Person1 > Teacher 没有在Student1里面所有false，兄弟不能跟父子一样
        System.out.println(object instanceof String);//false
        //Object > String    String跟Person1属于同类也不知Student1里面

        Person1 person = new Stunden1();
        System.out.println("================================");

        System.out.println(person instanceof Stunden1);//ture
        System.out.println(person instanceof Person1);//ture
        System.out.println(person instanceof Object);//ture
        System.out.println(person instanceof Teacher);//false
       // System.out.println(person1 instanceof String);//编译报错

        Stunden1 stunden1 = new Stunden1();
        System.out.println("================================");

        System.out.println(stunden1 instanceof Stunden1);//ture
        System.out.println(stunden1 instanceof Person1);//ture
        System.out.println(stunden1 instanceof Object);//ture
        //System.out.println(stunden1 instanceof Teacher);//编译报错
       // System.out.println(stunden1 instanceof String);//编译报错

 */

Student1

package com.oop.Demo05;

public class Stunden1 extends Person1 {
    
    

    public void go(){
    
    
        System.out.println("go");
    }

    @Override
    public void move() {
    
    
        System.out.println("学生跑了起来");
    }
}

Teacher

package com.oop.Demo05;

public class Teacher extends Person1 {

}